The present invention relates a bidirectional non-linear device, and more particularly to such a device for a liquid crystal display.
A bidirectional non-linear device such as a varistor represents a non-linear change in voltage-current characteristic and therefore has been discussed in recent years as a device for driving a liquid crystal display cell. As a device used for this purpose, a non-linear device with a metal-insulator-metal structure (called hereinafter as a "MIM device") has been proposed in Technical Digest of IEDM, pp 707-710, 1980.
This MIM device is composed of a lower metal electrode made of tantalum (Ta) formed on a glass substrate, an anodized tantalum oxide (Ta205) layer formed on the lower metal electrode, and an upper metal electrode made of chromium (Cr). When a voltage above a threshold level is applied between the lower and upper electrodes, electrons are transmitted therebetween the tunnel effect through the traps in the anodized oxide layer, so that the non-linear phenomenon is obtained in the voltage-current characteristic. However, the anodized oxide is a relatively porous, and hence the structural defects such as pinholes are easy to be formed in the anodized oxide film. The MIM device having the structural defects cannot represent the non-linear characteristic and thus cannot drive the liquid crystal display cell. Assuming that 1% structural defect is generated in MIM devices for driving one hundred thousand pieces of liquid crystal display cells, one thousand pieces of display cells are not driven. In other words, it is difficult to put the liquid crystal display employing MIM devices into practical use.